Mixed Halide Scintillators

ABSTRACT

A mixed halide scintillator material including a fluoride is disclosed. The introduction of fluorine reduces the hygroscopicity of halide scintillator materials and facilitates tuning of scintillation properties of the materials.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional ApplicationsSer. No. 61/673,487, filed Jul. 19, 2012, which provisional applicationis incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to scintillator materials and particularly tometal halide scintillator materials. Certain arrangements also relate tospecific compositions of such scintillator material and method of makingthe same.

BACKGROUND

Scintillator materials, which emit light pulses in response to impingingradiation, find a wide range of applications, including medical imaging,particle physics and geological exploration. While a varietyscintillator materials have been made, there is a continuous need forsuperior scintillator materials.

SUMMARY

The present disclosure relates to halide scintillator materials thatinclude two or halide anions, one of which is fluorine (F). Thecombination of fluorine and another halide results in a less hygroscopicscintillator material than a halide without fluorine.

Examples of halide scintillator materials with low-hygroscopicityinclude

DESCRIPTION

Cerium doped lanthanum bromide LaBr₃:Ce is an excellent scintillatorwith a significant flaw: It is hygroscopic. Hydrolysis producesoxyhalides, in this case LaOBr, which is a light scattering center. Thisproperty makes it extremely difficult to manufacture the raw materialswith sufficient purity. Moreover, crystals of halide scintillators mustbe grown in a moisture-free environment without the presence of oxygen.The reaction of halides scintillators with moisture makes cutting,polishing, and long-term sealing of detector assemblies very difficult.The final assembly must be hermetically sealed for years.

Certain examples disclosed here in of the invention use two or moredifferent halides in the same compound. Mixed halide anions can be usedto tune the properties of the resulting scintillator. The hygroscopicityof halides materials can be reduced significantly.

For example, lanthanum fluoride (LaF₃) is not hygroscopic and is quiteinsoluble in water. This is because fluorine is more electronegativethan oxygen and thus oxygen cannot displace fluorine. Oxygen is moreelectronegative than the other halides (Cl, Br, I), and these halidesare therefore easily hydrolyzed. The result is that LaF₃ is notsusceptible to hydrolysis. Thus, one can make stoichiometric compoundssuch as LaFBr₂ and LaF₂Br. The amount of fluorine can vary anywhere inthe range of 0 to 3 and can make non-stoichiometric compounds.

In LaBr₃, it is quite easy to make lanthanum oxybromide (LaOBr) byreaction with a water molecule. That is, the bromide is easilyhydrolysable. However, in LaF₂Br, the fluoride ion is not hydrolysable.The compound thus does not make the oxyhalide.

Additionally, one can make ternary compounds such as LaClBrI or evennon-stoichiometric quaternary mixtures such as LaFClBrI compounds. Itshould be noted that because of its small ionic radius, fluoridesusually make the densest compounds. The high density is desirable forradiation detection. This provides the opportunity to fine-tune theelectronic structures of the valence and conduction bands and theoverall efficiency of the scintillation mechanism.

Typically, the metal cation coordinates with many anions, usually nine.Using fluoride to stabilize the hydrolysis, one can use the otherhalides in the remaining sites to optimize the properties for lightoutput, decay time, density, energy resolution, and linearity.

Cerium represents interesting opportunities because it isself-activated. Cerium fluoride (CeF3) has a low light output, aboutone-half BGO. Cerium bromide (CeBr3) has a large light output of 68,000photons/MeV. Cerium chloride is itself a good scintillator. Thus, onecan use all four halides and fluoride to reduce hygroscopicity, and theothers to optimize light output and decay time.

Iodine is very difficult to use as a scintillator. It typically makesthe most soluble of the halides. It is also photochemically activebecause of the bond weakness. In the presence of oxygen and light,iodides react irreversible causing yellowing. A fluoride ion in thevicinity of an iodide could stabilize the photochemical activity of theiodide.

Bismuth halides are less hygroscopic than lanthanum halides. It is alsoa self-activated scintillator. Bismuth fluorohalides present anotheropportunity. The reduced hygroscopicity and the ability to be doped withcerium present advantages. Bismuth has the highest atomic number of thestable element.

This principle applies to any metal in the periodic table with a valenceof two or greater. There is a fair amount of information on BaFCl:Eu,which is non-hygroscopic. BaFI:Eu is also non-hygroscopic, made bymixing BaI2 in water (with europium iodide) and adding ammoniumfluoride. This precipitates insoluble BaFI:Eu This compound is ready forcrystal growth and produces 55,000 photons/MeV.

More generally, a scintillator compound can be male by making a solublemetal halide and adding ammonium fluoride until precipitation occurs.This ensures the compound is not hygroscopic.

Thus, metal halide scintillation materials with improved moistureresistance, density and/or light output can be made with the addition offluorine. Because many embodiments of the invention can be made withoutdeparting from the spirit and scope of the invent on, the inventionresides in the claims hereinafter appended.

1. A scintillator material, comprising mixed halides of ametallanthanum, comprising anions of fluorine and at least anotherhalide, the metal being lanthanum, cerium or bismuth.
 2. Thescintillator material of claim 1, comprising LaFBr₂ or LaF₂Br.
 3. Thescintillator material of claim 1, comprising LaFClBrI.
 4. Thescintillator material of claim 1, comprising mixed halides of cerium,comprising anions of fluorine and at least another halide.
 5. Thescintillator material of claim 1, comprising mixed halides of bismuth,comprising anions of fluorine and at least another halide.
 6. Thescintillator material of claim 1, comprising mixed halides of lanthanum,comprising anions of fluorine and at least one of chlorine, bromine andiodine.
 7. The scintillator material of claim 6, wherein the mixedhalides are doped with cerium.
 8. The scintillator material of claim 5,wherein the mixed halides are doped with cerium.
 9. The scintillatormaterial of claim 6, having a composition LaF_(x)A_((1−x)), wherein0<x<3.
 10. The scintillator material of claim 6, therein the otherhalide is Br.
 11. A method of making a scintillator material, comprisingmaking a compound of mixed halides with anions of fluorine and at leastone other halide, and selecting a composition of the material based on adesired combination of hygroscopicity and scintillation light output anddecay time.
 12. The method of claim 11, wherein the compound comprisesmixed halides of lanthanum, cerium or bismuth.
 13. The method of claim12, wherein making the compound comprises making a soluble metal halideand adding to the soluble metal halide ammonium fluoride untilprecipitation occurs.